The present invention relates, in general, to an air compressor that is powered by an electrical motor.
More particularly, the present invention relates to an air compressor, driven by an electrical motor, which is used to supply compressed air to the air brake system of a railed vehicle (e.g., a train or light rail vehicle).
Even more particularly, the present invention relates to a shaft extension which enables an air compressor that supplies compressed air for a braking system and that is driven by an electrical motor to be retrofitted such that the outboard end of the crankshaft of the compressor is supported by a xe2x80x9cthirdxe2x80x9d or xe2x80x9coutboardxe2x80x9d bearing. As explained fully below, the provision and use of such a xe2x80x9cthirdxe2x80x9d or xe2x80x9coutboardxe2x80x9d bearing significantly reduces the possibility that the rotor of the electrical motor will xe2x80x9ccantxe2x80x9d with respect to the stator of the electrical motor. Such relative angular displacement between the rotor and stator can significantly degrade the performance of the electrically powered air compressor, and can even lead to failure of the combined system.
The following background information is provided to assist the reader to understand the invention described and claimed herein. Accordingly, any terms used herein are not intended to be limited to any particular narrow interpretation unless specifically so indicated.
The use of an air compressor to supply compressed air for the operation of an air brake system is well known. In a railed vehicle, the air compressor is typically located in the locomotive of the train, etc. Earlier air compressors for trains were often powered via a power takeoff linkage from the engine of the locomotive. More modern diesel locomotives typically employ electric motors to supply tractive power, with the electrical power being generated onboard. The air compressors of diesel locomotives are, therefore, typically driven by electrical power, which is readily available onboard.
A main compressed air reservoir is normally employed. The main reservoir supplies compressed air to the xe2x80x9cbrake pipe,xe2x80x9d which runs the length of the train. The electric motor that drives the air compressor is typically started and stopped on an xe2x80x9cas neededxe2x80x9d basis, so as to maintain the compressed air pressure in the main reservoir within determined limits. Thus, the electric motor may be started and stopped repeatedly over the service life of the unit. FIG. 1 is a simplified isometric view of an air compressor unit that is widely employed within the railroad industry for supplying compressed air for use in air braking systems, namely, a xe2x80x9c3-CDxe2x80x9d Air Compressor manufactured by the Westinghouse Air Brake Company(copyright) division of Wabtec Corporation(copyright) (1001 Air Brake Avenue, Wilmerding, Pa.). Particulars of the xe2x80x9c3-CDxe2x80x9d Air Compressor are set forth in the pamphlet entitled xe2x80x9cInstructions for Disassembly, Repair and Assembly of xe2x80x983-CDxe2x80x99 Air Compressors,xe2x80x9d published by the above-identified Westinghouse Air Brake Company(copyright) (copyright 1994), this document being hereby expressly incorporated by reference into the present application, with the same effect as if fully set forth herein.
In FIG. 1, a xe2x80x9c3-CDxe2x80x9d air compressor is generally indicated by reference numeral 10. The air compressor 10 includes a crankshaft 12, which is driven by an external power source and which, in turn, drives the internal compression parts of the air compressor 10 (e.g., pistons, valves, etc.). The crankshaft 12 is rotationally supported and positioned by typically two inboard rotational bearings, one such inboard bearing 14 being shown in phantom in FIG. 1. The inboard bearing 14 is supported and positioned by a generally key-shaped bearing plate 16, which also serves to close a portion of the crankcase of the air compressor 10. It will be seen that the crankshaft 12 projects outward from and beyond the bearing plate 16.
FIG. 2 illustrates the manner in which an electric motor, generally indicated by reference numeral 18, has heretofore been mated with the air compressor 10, in order to provide power to the air compressor 10. The electric motor generally includes a stator frame 20, a stator 22, and a rotor 24. The stator frame 20 has, in the past, been connected to the exposed face of the bearing plate 16 by bolts 25 which pass through holes 26 provided in an inwardly projecting lip 28 provided on the rearward face of the stator frame 20. The bolts then engage a series of threaded blind holes 30 provided in the outwardly exposed face of the bearing plate 16. The stator frame is therefore xe2x80x9ccantileveredxe2x80x9d from the exposed face of the bearing plate 16 and secured in this position by the bolts.
The stator frame 20 may be viewed as the xe2x80x9chousingxe2x80x9d of the electric motor 18, serving to enclose the stationary stator 22 and the rotating rotor 24. The electric motor 18 is typically an induction type motor, and often a three-phase AC induction type motor. The stator 22 typically includes a plurality of coil windings and is fixedly mounted to the interior surface of the stator frame 20. The rotor 24 non-rotationally engages the protruding portion of the crankshaft 12 (i.e., is fixedly mounted with respect to the crankshaft 12) and is therefore encircled by the fixed stator 22. Typically, the rotor 24 is press fitted onto the crankshaft 12, and a protruding axial spline provided on the interior cylindrical surface of the rotor 24 engages a groove provided on the crankshaft 12.
An endnut 32 may engage a threaded portion 34 provided on the outboard distal end of the crankshaft 12 to axially retain the rotor 24 on the crankshaft 12. is The dimensional difference between the interior diameter of the stator 22 and the exterior diameter of the rotor 24 is relatively small, typically on the order of between about {fraction (40/1000)} and about {fraction (50/1000)} of an inch. If the rotor 24 is not maintained in a substantially central alignment with respect to the encircling stator 22, the rotor 24 may come into contact with the stator 22. Such rubbing degrades performance. In severe cases, contact of the rotor 24 with the stator 22 can short out the windings of the stator 22, thereby xe2x80x9cburning outxe2x80x9d the electric motor 18.
During startup of the electric motor 18, it has been discovered that a non-symmetric radial force is exerted on the rotor 24, and thus the crankshaft 12. Thus, during startup, forces are exerted on the rotor 24 which tend to xe2x80x9ccantxe2x80x9d the rotor 24 with respect to the stator 22. Over time in service, these forces can lead to the rubbing described above and, ultimately, can result in the above-described shorting and burning out of the electric motor 18.
There is disclosed in U.S. patent application Ser. No. 09/593,558, entitled xe2x80x9cLocomotive Air Compressor with an Electric Motor Supported by an External Bearingxe2x80x9d and in U.S. Ser. No. 09/593,559, entitled xe2x80x9cLocomotive Air Compressor with Motor Supported by Outside Bearingxe2x80x9d (both of these pending U.S. applications being assigned to the same assignee as the present application), various arrangements for providing what is herein referred to as a xe2x80x9cthirdxe2x80x9d or (alternatively) an xe2x80x9coutboardxe2x80x9d bearing. Such a third or outboard bearing provides additional support for the outboard distal end of the crankshaft 12, and considerably prevents (or at least substantially reduces) any canting of the crankshaft 12 and the rotor 24 attached thereto with respect to the stator 22.
There are an extremely high number of air compressors of the xe2x80x9c3-CDxe2x80x9d type in service. It is desirable, therefore, to provide an apparatus and method for xe2x80x9cretrofittingxe2x80x9d such in-service air compressors with such a third or outboard bearing. An apparatus and method for performing such a retrofit are disclosed herein.
Since relatively tight tolerances are required in the alignment between the stator frame 20 (which ultimately determines the positioning of the stator 22) and the crankshaft 12 (which ultimately determines the positioning of the rotor 24), it has heretofore been the practice in the industry to carefully machine both the outwardly exposed face of the bearing plate 16 and the rearward face of the stator frame 20 (i.e., including the inwardly projecting lip 28 provided on the rearward face of the stator frame 20) to relatively exact dimensions, in order to ensure that the rotor 24 remains rather exactly centered with respect to the stator 22.
Such precise machining of the previously separate bearing plate 16 and stator frame 22 is an expensive procedure, and is not always entirely satisfactory in its implementation. A combined bearing plate and stator frame which eliminates the need for separate machining of the bearing plate and stator frame separately to the aforementioned tight tolerances required, and which provides for substantially increased precise alignment of the rotor 24 within the stator 22 over use in service, is disclosed herein.
Therefore, one objective of the invention is the provision of a shaft extension which will enable an existing electric motor driven air compressor to be retrofitted such that the end of the crankshaft of the air compressor terminates in an outboard rotational bearing, which provides considerable structural support to the crankshaft and considerably reduces the possibility that the air compressor will suffer decreased performance or failure due to the rotor and stator of the electric motor having become canted with respect to one another.
Another objective of the invention is the provision of such a shaft extension which is inexpensive to produce and reliable in operation.
In addition to the objectives and advantages listed above, various other objectives and advantages of the invention will become more readily apparent to persons skilled in the relevant art from a reading of the detailed description section of this document. The other objectives and advantages will become particularly apparent when the detailed description is considered along with the drawings and claims presented herein.
The foregoing objectives and advantages are attained by the various embodiments of the invention summarized below.
In one aspect, the invention generally features a shaft extension for attachment to a crankshaft of an electric motor driven air compressor to axially extend the crankshaft for engagement with an outboard rotational bearing. The electric motor driven air compressor includes a bearing plate, an inboard rotational bearing mounted on the bearing plate, the crankshaft projecting through both of the inboard rotational bearing and the bearing plate and terminating in a distal end disposed exterior of the bearing plate. The air compressor also includes an electric motor, the electric motor having a stator frame, a stator fixedly mounted within the stator frame, and a rotor fixedly mounted to the crankshaft for rotation therewith and disposed interior of the stator. The air compressor further includes the outboard rotational bearing disposed axially outboard of the stator frame, the stator, and the rotor. The shaft extension includes an extension member having an extended major axis and a minor axis extending substantially transverse to the extended major axis and a connection mechanism for connecting the extension member to the distal end of the crankshaft.
In another aspect, the invention generally features, an improvement in combination with an electric motor driven air compressor. The air compressor has a bearing plate and an inboard rotational bearing mounted on the bearing plate, a crankshaft rotationally supported by the inboard rotational bearing, extending through the bearing plate, and terminating in a distal end disposed exterior of the bearing plate. The air compressor additionally includes an electric motor having a stator frame, a stator fixedly mounted within the stator frame, and a rotor fixedly mounted to the crankshaft for rotation therewith and disposed interior of the stator. The air compressor additionally includes an outboard rotational bearing disposed axially outboard of the stator frame, the stator, and the rotor. The improvement includes a shaft extension, which includes an extension member having an extended major axis and a minor axis extending substantially transverse to the extended major axis and a connection mechanism for connecting the extension member to the distal end of the crankshaft.